Bright-zinc plating bath

ABSTRACT

A BRIGHT-ZINC PLATING BATH IN WHICH THE PRIMARY BRIGHTENING AGENT CONSISTS OF A REACTION MIXTURE OF POLYAMINES, SALTS OF THE METALS OF GROUPS V AND VI OF THE PERIODIC TABLE AND SULFUR-RELEASING ORGANIC COMPOUNDS. THE SULFURSPLITTING COMPOUNDS ARE PREFERABLY ALIPHATIC AROMATIC AND HETEROCYCLIC COMPOUNDS CONTAINING SULFUR AND INCLUDE MERCAPTANE, THIOCOMPOUNDS AND THE LIKE.

United States Patent O1 zfice 3,572,971 Patented June 27, 1972 3,672,971BRIGHT-ZINC PLATING BATH Gerd Senge, Brackwede, and Rolf Sieburg,Eickum, Germany, assignors t Firma Riedel & Co., Bielefeld,

Germany No Drawing. Filed July 7, 1970, Ser. No. 53,038 Claims priority,application Germany, July 15, 1969,

P 19 35 821.5 Int. Cl. C23b 5/10 US. Cl. 204-55 R 6 Claims ABSTRACT OFTHE DISCLOSURE A bright-zinc plating bath in which the primarybrightening agent consists of a reaction mixture of polyamines, salts ofthe metals of Groups V and VI of the Periodic Table and sulfur-releasingorganic compounds. The sulfursplitting compounds are preferablyaliphatic aromatic and heterocyclic compounds containing sulfur andinclude mercaptane, thiocompounds and the like.

FIELD OF THE INVENTION Our present invention relates to a bright-zincelectrobathing bath, a method of making such a bath, and anelectrogalvanizing method for producing a bright-zinc coating and theproduct made by the latter method; more particularly, the inventionrelates to a bright-zinc electrogalvanizing system which eliminates manyof the disadvantages of earlier electrogalvanizing methods, includingtoxicity of the bath, the difliculty in processing the waste liquors,and the poor plating efficiency of those baths which are free from theother disadvantages.

BACKGROUND OF THE INVENTION In the electrogalvanization of metalsubstrates, it is possible to produce a bright-zinc coating using fiveconventional zinc-plating baths. These conventional baths include astrongly alkaline cyanide-containing bath, a strongly alkalinecyanide-free bath, a weakly alkaline pyrophosphate-containing bath, aweakly acid bath and a strongly acid bath. As will be apparenthereinafter, the choice among these baths has often been a compromise ofthe disadvantages, inconveniences and necessary evils inherent therein.For example, a strongly alkaline cyanidecontaining bath a strong acidbath have a common disadvantage in that it is difiicult to maintain thestability of the system during plating operation. Even with intensemonitoring of the alkali or acid concentrations, deviations are found tooccur which affect the nature of the zinc plate, the plating rate andother parameters of the system. The cyanide-containing zinc bath has thefurther disadvantage that the presence of cyanide salts renders the bathhighly toxic and makes it difficult to so treat the bath liquor as toenable its convenient disposal. Aside from the handling problems arisingwith the use of highly acidic zinc-plating baths, such baths are foundto have poor throwing power and to give poor coatings in many instances.

To avoid these major disadvantages in the most common types of platingbaths, it has been proposed to provide strongly alkaline cyanide -freeelectrogalvanizing baths, weakly alkaline pyrophosphate-containing bathsand weakly acidic high-conductivity baths, all of which are free fromthe toxicity characterizing the presence of cyanide compounds althoughthey are incapable of solving other problems which arise in thebright-zinc plating of metal substrates. The pyrophosphate-containingelectrolytes are disadvantageous in that the pyrophosphates and othercomponents thereof are recognized widely as waterpollutants even thoughthey may be present in small quantities. Hence the disposal of apyrophosphate-containing liquor is a major problem.

Furthermore, alkaline cyanide-free electrogalvanizing baths generallyinclude significant quantities of complexing agents or complex formerswhich are water pollutants of no small degree as well. Here again,disposal of the waste electrolyte is a major problem. Some of thesedifficulties are not encountered when Weakly acidic zinc-plating bathsare used although such baths require extensive pretreatment and aftertreatment, somewhat in the manner of high-conductivity nickel baths, atcorrespondingly high cost and effort.

In the cyanide-free alkali zinc-plating bath it has been proposedheretofore to provide brighteners or brightening agents which appear tobe effective in yielding a brightzinc coating instead of a coating witha matte-like appearance. Such conventional brighteners include gelatineand aldehydes (e.g. benzaldehyde, vanillin, heliotropin or piperonal andanisaldehyde or anisic aldehyde), compounds generally salts) of metalsof Groups VI and VII of the periodic system, especially chromium,molybdenum, tungsten and manganese, and generally compounds of theeighth Group of the Periodic Table, namely, iron, cobalt and nickel. Thedisadvantage of this system is that the gelatine and aldehydes areunstable in a strongly alkaline medium and sutfer alkali-hydrolysis,therefore rendering the system practically inutile for commercialoperations.

Much may also be made of the fact that it is known to provide in zincelectroplating bath, complex formers or complexing agents such astriethanolamine, polyethylenamine, ethylenediaminetetraacetate,triisopropanolamine, dimethylamine, polyamines (such asdiethylenetriamine, triethylenetetramine and pentaethylene) andhexamethylenetetramine. Complexing agents of this type have theadvantage that they stabilize the stability of the electrolyte althoughthey often give rise to the problem of treatment of the wasteelectrolyte. This is all the more significant when it is recognized thatzinc and other toxic heavy metals may be solubilized as a result of thepresence of the complex formers and remain in the electrolyte as thelatter is treated as a waste efiluent.

There are also extant brightening systems for brightzinc electroplatingwhich are combinations of ketones (for example, cyclohexanone anddiphenylketone) and polyamines, which have been found to be mosteffective in the higher current-density ranges for producing bright-zinccoatings. A disadvantage of such systems, probably based upon theketone-component, is that in short order there is a reduction in thebrightness and a noticeable reduction in the attainable current density.While investigations have shown that the combination of polyamines withbrighteners of the aldehyde type are also effective in theelectrodeposition of bright-zinc coatings, the current-density rangeover which the bright coating is produced, is proportionally narrow and,therefore, difiicult to control.

OBJECTS OF THE INVENTION It is, therefore, the principal object of thepresent invention to provide a bright-zinc electroplating orelectrogalvanizing bath which is free from the difliculties of disposal,is nontoxic, and can produce bright-zinc coatings over a relatively widecurrent-density range at high efficiency and with good throwing power.

Another object of this invention is to provide an improved bright-zincelectrogalvanizing bath which avoids the difficulties hithertoencountered with conventional baths and yet is able to produce anexcellent bright-zinc plating or coating in a highly stable manner withhigh efliciency.

Still another object of our invention is the provision of an improvedmethod of making a bright-zinc electroplating bath which permits highlyefficient zinc plating of a bright or shiny coating with high throwingpower and current density and without the toxic difliculties encounteredwith some of the earlier baths proposed for bright-zincelectrogalvanizing.

It is also an object of our invention to provide an improved method ofelectroplating metal substrates to yield bright-zinc coatings.

Another object of our invention is found in a plating body in which theabove methods and improved plating systems are employed.

The invention also has as its object the provision of an improvedbrightening composition or brightening agent for bright-zincelectrogalvanizing baths.

SUMMARY OF THE INVENTION These objects and others, which will becomeapparent hereinafter, are attained, in accordance with the presentinvention, by the provision of an aqueous alkali cyanidefree bright-zincelectrogalvanizing or electroplating bath which includes as abrightening system, agent or composition a combination of at least threecomponents and, preferably, a reaction mixture thereof, the threecomponents being a polyamine (of the type mentioned earlier and hithertoused as a component of brightening agents for zinc electroplates baths),a salt soluble in the alkali medium of at least one of the metals ofGroups V and VI of the Periodic Table and preferably a vanadate or amolybdate, and a sulfur-splitting organic compound, i.e. an organiccompound containing sulfur from which the sulfur is readily released.

When reference is made herein to a reaction mixture, it is to beunderstood that we have found that a mixture of the three componentsappears to undergo a chemical reaction of the complex-forming or liketype. In other words, chemical bonds are created when the threecomponents are combined, which bonds are not found in the threecomponents individually. It has been discovered that it is advantageousto combine the three components prior to their introduction into thebath to obtain maximum benefit from the interaction observed above,although the effect of the brightening system is noticeable to a lesserextent when the three components are introduced individually into theelectroplating electrolyte. Reference to the Periodic Table is intendedto mean the chart of the periodic arrangement of the elements containedon pages 445 and 446 of the Handbook of Chemistry and Physics, 41edition, 1959-1960 or the periodic chart of the elements contained onpages 448 and 449 of this publication. When the latter chart isemployed, the groups to which the metals should belong are Groups V-Band VI-B. More specifically, the applicable metals are vanadium,chromium, niobium, molybdenum, tantalum and tungsten, althoughpreference is given to vanadium and molybdenum, as discussed above.

The sulfur-splitting compounds most suitable for the purposes of thepresent invention are those which react most readily with the othercomponents of the reaction mixture, as defined above. These compoundsinclude mercaptans and thio-compounds and are generally describable asthe aliphatic, aromatic, aromatic-heterocyclic compounds readilyyielding sulfur. The compounds which have been used, in combination orindividually, include mercaptothiazoline, mercaptopyridine (C H NSH)thioureas and derivatives thereof, dithiocarbamic acids and salts andderivatives thereof, thioacetamide and thiocarbanilide(diphenylthiourea) According to a more specific feature of thisinvention, the reaction mixture is reacted at a temperature of about 20C. in an initial reaction stage and is thereafter subjected to asecond-stage reaction at a higher temperature in the presence of hotwater. Preferably, the second stage of the reaction or the afterreaction is effected at a temperature of 70 C., i.e. at the temperatureof the hot water which is introduced into the reaction mixture or 4 viceversa. Following the second reaction stage, the temperature of thereaction mixture is slowly cooled to a temperature of about 20 C.Advantageously, the reaction mixture is cooled progressively to thetemperature of 20 C. in the presence of formaldehyde or sodiumhydroxide.

According to another feature of this invention, the reaction mixture ispresent in the electrolyte or plating bath in an amount ranging between0.01 to 10 parts by weight per parts by weight of the remainder of thebath, although these limits are not critical and it is possible merelyto add the reaction mixture in the cyanide-free alkaline plating bathuntil the desired quality of brightplating is obtained. Similarly theratios of the three components may range widely and, for example, thepolyamine may be present in an amount (by weight) ranging between 1 and20 times that of the sulphur-containing compound, while thesulphur-containing compounds may be present in an amount (by weight)ranging between 0.5 and 20 times that of the metal salt.

An important advantage of the electrolyte of the present invention isthe low proportion of complex formers therein and, consequently, theease with which the waste liquor can be treated or disposed of. Adilution of 1:100 yields a complexed-zinc concentration which isrelatively low, i.e. below 3 mg. per liter.

In the following examples of compositions of the zinc electroplatingbath, according to the invention, bright plating of zinc was carried outin each case at a temperature of 10 to 30 C., at intervals of 2 C., atcurrent densities of 0.5 to 4 amperes per/dm. for a treatment time of 10to 20 minutes. The treatment times were varied at two-minute incrementsand the current density tests were taken with 0.5 ampere per/dm.increments. In each case, the composition yielded a bright zinc coatupon a metallic substrate electrogalvanized in the bath.

Example I Grams/liter Zinc chloride 50.0 Sodium hydroxide 230.0Tetraethylenepentamine 2.0 Ammonium molybdate 0.1 Sodiumdimethyldithiocarbamate 0.1 Heliotropin or heliotropin-bisulphite 1.0Water Balance Example II Grams/liter Zinc oxide 30.0 Sodium hydroxide200.0 Pentaethylenehexamine 4.0 Thiourea 0.2 Mercaptothiazoline 1.0Sodium diethyldithiocarbamate 0.1 Ammonium molybdate 0.28 35% aqueoussolution of formaldehyde 0.4 Vanillin 1.0 Water Balance Example IIIGrams/liter Crystalline zinc sulphate 130.0 Sodium hydroxide 250.0Tetraethylenepentamine 2.0 Thiourea 1.0 Triethylenetetramine 1.5Thioacetamide 1.0 Ammonium vanadate 0.5 Formaldehyde (35% aqueoussolution) 0.5 Piperonal 0.4 Vanillin 0.4

Example IV Grams/liter Zinc oxide 30.0 Sodium hydroxide 180.0Triethylenetetramine 2.0 Ammonium molybdate 0.2 4-mercaptopyridine 0.5Formaldehyde (35% aqueous solution) 0.5 Piperonal 0.4 Vanillin 0.4

It has been found, surprisingly, that the use of the individualcomponents of the reaction mixture of the present invention, i.e.without prereaction, as described generally above, is significantly lesseffective in providing a brightzinc coating than the same compositionwherein, however, the polyamine, the sulphur-containing organic compoundand the metal salt are prereacted prior to introduction into theelectrolyte. It appears that the prereaction gives rise to ametalsulphidoamino complex of an as yet undefined composition, whichnevertheless is the effective species in promoting the formation ofuniform bright zinc coatings within the wide range of current densitiesand with a high tenacity to the substrate. In fact, thecurrent-efliciency curve of electrogalvanic metallic substrates withbright zinc produced from the baths of Examples LIV, above,appproximates the curves obtained with bright zinc coating from cyanidebaths, without however the toxicity associated therewith. This highadvantageous currentefliciency characteristic is not, however, obtainedwhen the substances of the reaction mixture are supplied to theelectrolyte individually, i.e. without a preliminary reaction. Whereasthe prereacted mixture provides an extremely bright coating, the use ofthe same quantities of the individual components yields a bright coatingwhich is less brilliant, is striated and is flakey.

It will be understood that, as illustrated in the examples, the reactionmixture can be employed together with conventional brighteners asenumerated above. While, as pointed out, the mechanism time which theactive species of the reaction mixture is formed is not fully clear, ithas been found that deviations from the proportions given above andreaction steps under markedly different conditions do not yield theactive species or a reaction mixture which is optimally elfective in theformation of bright zinc coatings. In each of the aforementionedexamples, the polyamine, the sulphur-containing organic compounds andthe metal salts are reacted at about room temperature, i.e. 20 C. bybeing brought together for a period of a few minutes and intimatelymixed. Thereafter, about 20 ml. of water at 70 C. is introduced intoeach reaction mixture (second stage reaction) whereupon formaldehyde andcaustic soda is added (about 5 grams for the amounts indicated of thereaction mixture). The formaldehyde appears to be an accelerator whilethe caustic soda functions as a stabilizer. When the formaldehyde is notsupplied, the final reaction takes substantially longer. In each case,the electroplating bath is formed by dissolving the zinc salt and thesodium hydroxide in water and adding the reaction mixture to make up thespecified compositions. Thereafter, any customary brighteners, if any,may be supplied.

We claim:

1. A method of making an electrolyte for the brightzinc plating of asubstrate comprising the steps of interreacting at least one organicpolyamine selected from the group which consists of diethylenetriamine,triethylenetetraamine, pentaethylenehexamine, hexamethylenetetramine,and ethylenediamine; at least one aliphatic, aromatic oraromatic-heterocyclic sulphur-containing compound selected from thegroup which consists of mercaptothiazolines, mercaptopyridines,thioureas, dithiocarbamic acids and salts thereof, thioacetamides andthiocarbanilids; and at least one vanadate or molybdate salt andincorporating the resulting reaction mixture into an alkalineelectrolyte containing zinc.

2. The method defined in claim 1 wherein said reaction mixture issubjected to an initial reaction step at a temperature of about 20 C.,is thereafter treated with water at a temperature of about 70 C. in asecond reaction stage and is cooled to a temperature of about 20 C.slowly.

3. The method defined in claim 2 further comprising the step of treatingthe reaction mixture with formaldehyde and sodium hydroxide during thecooling thereof.

4. An electrolyte for the bright-zinc plating of a substrate as made bythe method defined in claim 1.

5. A method of brighbzinc electroplating a substrate comprising the stepof electrodepositing zinc from the electrolyte prepared as defined inclaim 1 at a current density of 0.5 to 4 amperes per dm. at atemperature of 10 to 30 C. for a period of 10 to 20 minutes.

6. An aqueous electrolyte as made by the method defined in claim 1 forbright-zinc electroplating and selected from the group which consistsof:

Grams/liter Zinc chloride 50.0 Sodium hydroxide 230.0Tetraethylenepentamine 2.0 Ammonium molybdate 0.1 Sodiumdimethyldithiocarbamate 0.1 Heliotropin or helioptropin-bisulphite 1.0Water Balance Grams/liter Zinc oxide 30.0 Sodium hydroxide 200.0Pentaethylenehexamine 4.0 Thiourea 0.2

Mercaptothiazoline 1 .0 Sodium diethyldithiocarbamate 0.1 Ammoniummolybdate 0.2 35% aqueous solution of formaldehyde 0.4 Vanillin 1.0Water Balance Grams/liter Crystalline zinc sulphate 130.0 Sodiumhydroxide 250.0 Tetraethylenepentamine 2.0 Thiourea 1.0Triethylenetetramine 1.5 Thioacetamide 1.0 Ammonium vanadate 0.5Formaldehyde (35% aqueous solution) 0.5 Piperonal 0.4 Water BalanceGrams/liter Zinc oxide 30.0 Sodium hydroxide 180.0 Triethylenetetramine2.0 Ammonium molybdate 0.2 4-mercaptopyridine 0.5 Formaldehyde (35%aqueous solution) 0.5 Piperonal 0.4 Vanillin 0.4- Water BalanceReferences Cited FOREIGN PATENTS 1,091,171 11/1967 Great Britain 204-55R 1,043,618 9/1966 Great Britain 204-55 R F. C. EDMUNDSON, PrimaryExaminer

